Ultrasonic imaging technology has become a vital tool for examining the internal structure of living organisms. For the diagnosis of various medical conditions, ultrasonic imaging is often useful to examine soft tissues within the body to show the structural detail of internal tissues and fluid flow.
To examine internal body structures, ultrasonic images are formed by producing very short pulses of ultrasound using a transducer, sending the pulses through the body, and measuring the properties (e.g., amplitude and phase) of the echoes from tissues within the body. Focused ultrasound pulses, referred to as "ultrasound beams", are targeted to specific tissue regions of interest in the body. Typically, an ultrasound beam is focused at various steps within the body to improve resolution or image quality. Echoes are received by the transducer and processed to generate an image of the tissue or object in a region of interest. The resulting image is usually referred to as a B-scan image.
Measuring and imaging blood (and other bodily fluid) flow within a living subject is typically done using the Doppler principle, in which a transmitted burst of ultrasound at a specific frequency is reflected from moving blood cells, thereby changing the frequency of the reflected ultrasound in accordance with the velocity in the direction of the flow. The frequency shift (Doppler shift) of reflected signals with respect to the transmitted signals is proportional to the velocity of the fluid flow. This frequency may be detected and displayed on a video display device to provide graphic images of moving tissue structure and fluid flow within a living patient.
Present ultrasound techniques include single-plane or bi-plane duplex Doppler imaging of tissue motion, as well as cross-correlation ultrasound estimation of displacements and mean velocities for color mapping tissue motion (referred to as CVI and developed by Philips Corporation).
FIG. 1 is a diagram of a prior art single-plane duplex Doppler imaging method. In the single-plane duplex Doppler imaging method, a vessel 154 is scanned in the longitudinal plane by an ultrasound transducer 152 using duplex Doppler methods to manually measure vessel diameter, d. This method relies upon a range-gate value and a time-averaged angle-corrected mean velocity in the spectral Doppler waveform. The vessel diameter is then used to compute the cross-sectional area of the region-of-interest to determine the volume flow through the region.
Another present ultrasound technique for computing volume flow is bi-plane duplex Doppler imaging for vascular volume flow. In this method, a vessel is scanned in the transverse plane using B-scan techniques to manually measure the vessel cross-sectional area. The vessel is then scanned in the longitudinal plane with duplex Doppler to place the range-gate and to measure a time-averaged angle-corrected mean velocity in the spectral Doppler waveform. The volume flow is then computed using these values.
An alternative bi-plane duplex Doppler imaging technique is used for measuring cardiac output. For this method, a left ventricular outflow tract (LVOT) is scanned in the transverse plane with B-scan to manually measure cross-sectional area (ACS). The LVOT is then scanned in the longitudinal plane with duplex Doppler to place the range-gate and measure the velocity-time integral (VTI) and heart rate (HR). The cardiac output is then computed using the equation CO=VTI.times.ACS.times.HR. The cardiac output (CO) serves as a measure of volume flow.
In the Color Motion-mode (M-mode), non-Doppler correlation method (CVI-Q of Philips), color M-mode data is used to provide continuous sampling of vessel diameter. A non-Doppler correlation method is used to determine mean flow velocity. The vessel diameter is then used to compute cross-sectional area. The time-averages of cross-sectional area and mean velocity provide the data to compute the volume flow.
Although well established, these present known methods are generally subject to error because of simplifying assumptions about several factors regarding the vessel through which the fluid (typically, blood) is flowing. For example, roundness of the vessel, temporal invariance of the vessel cross-section, and velocity invariance throughout the vessel cross-section are often assumed. Such factors, however, may vary widely from one application to another. Other sources of error associated with one or more of the cited prior art methods include biased Doppler sampling in a frequency-domain spectrum, error in manual Doppler angle correction, or error in manual tracing of a vessel perimeter.